1. Field of the Invention
The present invention relates to a method of forming a wiring layer which is applied to the manufacture of the semiconductor device, and more particularly to a method of forming a wiring layer in which a contact hole or via-hole having a large aspect ratio is filled with metal including aluminum.
2. Description of Related Art
The width of a wiring layer becomes thin as a semiconductor circuit has been fabricated to achieve higher density integration. Also, the diameter of the opening of a contact hole or via-hole (to be generally referred to as "a connection hole", hereinafter) is made small. The connection hole is formed in an interlayer insulating film which is interposed between a silicon substrate and the wiring layer, or between an upper wiring layer and a lower wiring layer. As a result, the aspect ratio of the connection hole, i.e., a ratio of the depth of the connection hole to the opening diameter of the connection hole exceeds "1".
Generally, a sputtering method is used to form the wiring layer. However, the sputtering method cannot achieve step coverage sufficient to fill the connection hole which has the high aspect ratio as mentioned above. There is a problem in that a wiring layer is broken at a step. Therefore, as the method of filling the connection hole which has the high aspect ratio, a CVD-W method which has good step coverage is often used at present instead of the sputtering method. However, there is a problem in that this CVD-W method is high in cost and generates cohesion of particles in many positions, compared to the conventional sputtering method.
For this reason, various methods are proposed in which the sputtering method is used which has a good productivity and the step coverage is improved. In these methods, an aluminum (Al) sputtering reflow method is employed in which, after Al is sputtered at a low temperature in a range of the room temperature to about 150.degree. C., a wafer is heated to 450 to 500.degree. C. without breaking the vacuum state to fill the connection hole with the sputtered Al. Alternatively, a high temperature sputtering method is employed in which, after Al is sputtered at a low temperature 1/3 to 1/2 of a desired film thickness, Al is sputtered for the remaining film thickness while a wafer is heated to 400 to 450.degree. C., such that the connection hole is filled with the sputtered Al.
However, in these methods, the state of the connection hole is greatly dependent on the coverage of Al in the initial stage of the filling process. For example, as shown in FIG. 3, because the coverage of an Al film 2 in the initial stage is wrong when the aspect ratio of a contact hole 1 exceeds "1", there is a problem in that the Al film 2 is broken on the side wall portion of the contact hole 1 when the wafer is heated to high temperature. Also, a void 3 is formed in the contact hole 1, so that the contact hole which is not sufficiently filled with Al is formed.
There are proposed some methods in which the problem that the void is generated in the connection hole can be prevented and the connection hole having the aspect ratio exceeding "1" can be filled with Al.
The first of the these methods is a method of filling with Al alloy which is disclosed in Japanese Laid Open Patent Disclosure (JP-A-Heisei 6-275555). This method will be described with reference to FIGS. 1A to 1C.
First, as shown in FIG. 1A, after a connection hole 6 is formed in an insulating film 5, a titanium (to be referred to as "Ti" hereinafter) film 7 and a titanium nitride (to be referred to as "TiN" hereinafter) film 8 are deposited in the connection hole 6 as a laminate barrier metal film 9 which contains titanium system material. Next, as shown in FIG. 1B, a Ti film 10 is deposited on the laminate barrier metal film 9 by a collimate sputtering method. Subsequently, as shown in FIG. 1C, the connection hole 6 is filled with an Al film 11 in the same vacuum state without breaking the vacuum state using an Al reflow method or a high temperature sputtering method. In this method, wet property with Al is improved by using the TiN film 8 as a lower layer for the film 11 of Al system material. Thus, there is an effect that the cohesion of Al particles in high temperature processing is suppressed.
The second of the above methods is the method of forming a wiring layer which is disclosed in Japanese Laid Open Patent Disclosure (JP-A-Heisei 4-280425). This method will be described with reference to FIGS. 2A to 2D. First, as shown in FIG. 2A, after a connection hole 6 is formed in an insulating film 5 to extend a diffusion layer, a Ti film 13 is deposited in the connection hole 6 as the first layer barrier metal film. Next, as shown in FIG. 2B, a TiON film 14 is deposited on the Ti film 13 by a bias sputtering method as second layer barrier metal film. Alternatively, after the TiON film 14 is formed by a usual sputtering method, laser anneal is performed. The TiON film 14 formed on Ti crystal grains 15 in this way is composed of TiON crystal grains 16 each of which has a large crystal grain size and a flat surface, as shown in FIG. 2D. Subsequently, as shown in FIG. 2C, the upper portion of the connection hole 6 is filled with an Al film 17 using a high temperature sputtering method. This method has an effect that the TiON film 14 is formed as the barrier metal to have the flat surface, so that a wet property of the TiON film with the Al film is improved and the fluidity of Al is improved.
The third of the above methods is disclosed in Japanese Laid Open Patent Disclosure (JP-A-Heisei 5-29251). In this method, after a connection hole is formed, a TiW or TiN film is formed. When the connection hole is filled with Al at a high temperature, the filling is performed while the contact hole is vibrated with supersonic wave of 1 MHz under high pressure of about 10 Torr. The method has an effect that the fluidity of Al is improved by the high pressure and the supersonic wave.
However, in the above-mentioned conventional methods, there are some problems for the purpose that the connection hole having, especially, an aspect ratio exceeding "1" is stably filled.
In the first method, the process of forming the Ti film as a lower layer for the Al system material film and the process of filling the connection hole with Al must be performed in the same vacuum state without breaking the vacuum state. Therefore, the process of sputtering Ti must be added before the process of filling the connection hole with Al. For this reason, the number of wafers processed per a unit time in the sputtering step (the throughput) reduces. Further, because a Ti collimate sputtering method is used, there is a problem in that the productivity is decreased. In addition, the use of the collimate boards directs the generation of particles. Also, because alloy such as Al.sub.3 Ti is formed from Ti and Al system materials at a high temperature, there is a problem that the contact/via resistance and wiring resistance increase when this alloy exists in the manner of islands.
In the second method, in a case of the connection hole having the aspect ratio of 1 or below, because the Al film can have sufficient coverage on the side wall of the connection hole and the barrier metal is smooth, it is possible that Al flows to fill the connection hole. However, in a case of the connection hole having the aspect ratio of 1 or above, because the Al film has wrong coverage on the side wall of the connection hole and the surface of the barrier metal is smooth when the Al film is heated to a high temperature, Al particles cohere so that the Al film is broken on the step of the side wall portion. As a result, there is a problem in that Al can not flow into the connection hole such that voids are formed in the connection hole.
In the third method, because the high pressure and the supersonic wave are used for filling the connection hole with Al, there is a problem in that a special unit is required which is different from the conventional sputtering apparatus and the productivity is low.